So basically this guide will explain why Thresh is the best.
Thresh is basically the Lee Sin of support. He has a little bit of everything, and can be annoying in almost any situation. He has a hook/engage, a shield/saveyourdumbassADC, a auto attack poke enhancer/displacement, and AOE high damage/multitarget CC. The only thing he doesn't really have are heals/speed boost, which is where his items come in. First lets look at runes and masteries. Keep in mind, this is just what I use, variations may apply.
So you got your basic:
9x Greater Seal of Armor - For tanking autos from ADC
9x Greater Gylph of Magic Resist - For taking harass from supports/ nonAD adc skills
9x Greater Mark of Health - You know, because health
3x Greater Quintessence of Attack Damage - For thresh's mean auto harass.
Q - Death Sentence: You should be maxing this first because it is your main intiating/disrupting spell to use. The increase in damage, and decrease of cool down is extremely important.
W - Lantern: You want to get a point in this at 3, stop prevent early ganks, shield your ADC from harass and early intiations.
E - Flay: Flay.
R - The Box: This skill is the hardest to place, because you need to get into the middle of your enemy team for it to be 100% effective, or use it to zone enemy carries out of fights.
This is where the most customizatibility comes into play.
This is what my typical build looks like by the end of the game for me. But there are many variations that different people prefer when playing, and most of them are situational.